1. Field of the Invention
This invention relates to imaging in optical code reading devices. More particularly, the present invention relates to a system and method for auto-focusing an image utilizing the principles of axial chromatic aberration.
2. Description of the Related Art
Auto-focusing systems in conventional cameras, optical code readers, etc. generally employ devices to determine the distance between the auto-focus device and the selected target or object before moving one or more lenses for focusing the object. Typically, such devices utilize the triangulation technique for measuring distances.
An image sensor based on the triangulation technique comprises an illumination device, such as an LED or a laser, and a detector, such as CCD. The illumination device projects a light-dot on the object. A proportion of the light reflected from the object's surface passes through a lens situated in front of the camera, optical code reader, etc. and onto a number of sensing elements in the camera. The angle at which the reflected light projects onto the sensing elements is established and the distance between the sensor and the object is determined through a number of calculations. One or more lenses are then automatically moved in accordance with the measured distance for auto-focusing the camera, optical code reader, etc.
This triangulation technique is typically prone to errors due to parallax, especially for nearby objects, as is generally the case for optical code readers used to image optical codes. Parallax is a change in the object's apparent position due to a change in the position of the sensor.
Another distance measuring technique utilizes the principle of image frequency content analysis. A distance measuring system employing image frequency content analysis impinges a beam on an optical target and captures the reflected beam for processing by an image sensor having an array. The principle of image frequency content requires the system to measure the value of each pixel within the predetermined array and compare that value to the value of adjacent pixel to determine the difference in pixel values.
The process repeats for all the pixels within the array where the sum of all the differences in pixel values is determined as an absolute value, thereby indicating a level of contrast in the array. Generally, the higher the measured contrast is, the sharper (i.e. more focused) the image of the optical target. However, a system employing this principle necessitates additional processing steps to calculate the differences in pixel values and determine the level of contrast in the image of the optical target. In such a system, the necessity of additional processing steps limits the response time of the system.
Accordingly, a need exists for a system and method for auto-focusing an image that are not prone to parallax errors, provide instantaneous or real-time focusing of an image, and configurable and adaptable for incorporation in various devices, such as cameras and mobile and stationary optical code readers.